Over the past several years, studies concerning foam-based packaging obtained from biorenewable materials have focused on the utilization of starch. There are four basic methods used so far for manufacturing the biodegradable packaging materials having a porous-foamed structure, namely: extrusion, baking process, compression and compression-explosion process.
A well known method for making such packaging involves making an aqueous suspension of starch with suitable additives known in the art, heating the suspension in a two-clamp mould. Shogren et al., POLYMER 39:6649-6655 (1998); Lawton et al., IND. CROP. PROD. 19:41-48 (2004); and Salgado et al., J. FOOD ENG. 85:435-443 (2008). The only foaming agent typically employed is evaporating water. The products of this type are manufactured in systems with a high moisture content (70-80%), therefore, the baking process itself is significantly longer (1-2 min) than the production of similar packaging from polystyrene. The longer time cost is the principal disadvantage of this method.
A similar method can be utilized for manufacturing foams from other biodegradable materials like, for example, polypropylene carbonate), i.e., a copolymer formed from propylene oxide and carbon dioxide (Luinstra, POLYMER REVIEWS 48(1):192-219 (2008)) with N,N′-dinitrozobenzmethylene used as a porophor and the addition of urea as a decomposing activator (Guan et al., J. POLYM. RES. 14: 245-251 (2007)).
Cellulosic/starch packaging materials with a three-layer structure are currently being produced by Novamont SpA in Novara, Italy. For example, Novamont produces a starch polymer that is formed into a corrugated foam sheet. Chynoweth and Gordon, PLASTICS TECHNOLOGY: 49(8):14 (2003). Such corrugated cardboards with foam based on the biodegradable polymers derived from starch (which are referred to as “Mater-Bi foam”) have been introduced on the European market in year 2002 in the form of a product called “Wave Mater-Bi”. The closed-cell foam is formed as a result of foaming by means of water steam in an unconventional extrusion line engineered by Novamont. This method produces the compositions being directly foamed in the stage of extrusion.
U.S. Pat. No. 4,435,344 describes a method for producing the composites by coating paper with a resin or laminated with thermoplastic films. A surface of such material is then heated and joined with a foaming layer of heat-insulating polyethylene. U.S. Pat. Nos. 6,030,476 and 5,840,139 describe a method for producing beverage containers or cups, where the insulating layer is formed by the utilization of thermoplastic synthetic resin. A description of the invention found in the '476 patent provides a heat-insulating material containing a thermoplastic synthetic resin with a low or medium density, for example polyethylene, polyolefin, nylon and a high-density polyethylene layer impermeable to moisture, located on the paper surface. The materials having a similar construction with the heat-insulating layer, made of the synthetic materials are described in European patents EP 0940240 A2 and EP 1060879. U.S. Pat. No. 7,074,466 describes a method for manufacturing a composite material having the heat-insulating properties, which is based on a cardboard layer and an expended foam layer located on the cardboard substrate.
The heat-insulating layers comprise the synthetic materials such as polyvinylidene chloride (“PVDC”) or acrylonitrile/methyl methacrylate (“AMM”) copolymer foams. This layer is formed at the stage of its coating. The well-known foamed biopolymer materials, primarily starch-based, regardless the used methods of manufacturing are characterized by a low flexibility and a relatively high susceptibility to absorb water steam. Moreover, in the state of being moist, after a long-term of conditioning in the environment with elevated moisture, the other properties of foamed starch undergo deterioration, i.e., the resistance to breaking or tearing, therefore, the attempts to incorporate a number of additions improving the functional properties of the material are still ongoing.
Among numerous attempts described in the research papers, the following should be mentioned: (1) the application of plasticizers to improve flexibility, where the primary plasticizers used for starch include glycerol, polyvinyl alcohol (Finkenstadt and Willett, CARBOHYD. POLYM. 55: 149-154 (2004)), ammonium chloride, sorbitol (Poutanen and Forssell, TRENDS IN POLYMER SCI. 4(4):128-132 (1996); Funke et al., POLYM. DEGRAD. STABIL. 59:293-296 (1998); (2) incorporation of fillers, such as: aspen fibres (Glenn et al., IND. CROP. PROD. 14:201-212 (2001a); Shogren et al., IND. CROP. PROD. 16:69-79 (2002); Lawton et al., supra), clays (Wilhelm et al., CLAY. CARBOHYD. POLYM. 52:101-110 (2003)) and chalk (Glenn and Orts, IND. CROP. PROD. 13:135-143 (2001)); and (3) the use of additives leading to the hydrophobization of starch: caprolactones, such as polycaprolactone (“PCL”) (Preechawong et al., POLYM. TEST. 23:651-657 (2004)), polylactic acid (“PLA”) (Preechawong et al., CARBOHYD. POLYM. 59:329-337 (2005)). Such prepared materials are frequently treated as hybrids.
Other auxiliary agents have also been used, such as anti-lumping substances (magnesium stearate) or those facilitating maintenance of the gas in the bubbles (e.g., guar rubber) (Shogren et al., supra). In the case of first commercial biodegradable packaging available on the market, protective coatings are frequently applied, the primary task of which is the protection of a hydroscopic biopolymer foam forming the fittings against moisture absorption and the improvement of the mechanical properties (Glenn et al., IND. CROP. PROD. 14:125-134 (2001b)).